Signal translating apparatus



Dec. 11, 1945. o so 2,390,847

S IGNAL TRANSLATING APPARATUS Filed Aug. 13, 1941 3nnenkor HWF 0190??? Patented Dec. 11, 1945 SIGNAL TRAN SLATING APPARATUS Harry F. Olson, Haddon Heights, N. J., asslgnor to Radio Corporation of America, a corporation oi Delaware Application August 13, 1941, Serial No. 406,597

Claims. (Cl. 1810.5)

This invention relates to signal translating apparatus, and more particularly to a microphone which is especially useful in submarine signalling.

Sub-aqueous microphones have been provided heretofore in which the signal translating unit is enclosed within a rubber casing through which acoustical vibrations may be transmitted from the water to the active element of the microphone. In such microphones, however, the rubber casing has been made relatively thick and stiff in order to withstand the pressure of the water at relatively great depths. As a result, -the rubber casing is incapable of faithfully transmitting to the active element of the microphone vibrations at acoustic frequencies, and such microphones are, therefore, relatively insensitive.

The primary object of my present invention is to provide an improved sub-aqueous microphone which is free from the aforementioned disadvantage.

More particularly, it is an object of my present invention to provide an improved sub-aqueous microphon which is efiiciently responsive to acoustical vibrations over the entire audible range.

Another object of my present invention is to provide an improved sub-aqueous microphone as aforesaid, the characteristics of which are independent of the depth to which the microphone may be immersed in the water.

Still another object of my present invention is to provide an improved sub-aqueous microphone as aforesaid in which the stiffness of the vibrating system is maintained at a relatively low level.

It is also an object of my present invention to provide a sub-aqueous microphone as set forth above which is simple in construction, inexpensive in cost, and highly efficient in use.

In accordance with one form of my invention, I provide a fluid-tight rubber tube or casing in which is enclosed a dynamic or other suitable electro-mechani-cal translating unit. The area of the rubber wall or casing presented to the water is large compared to the area of the diaphragm or other moving element of the translating unit. This tends to match the relatively large acoustic impedance of the water to the smaller acoustic impedance of the translating unit. In order to maintain a relatively low stiiiness in the vibrating system, I provide an air chamber which is coupled to the chamber housing the translatin unit and provide a communicating passage between the chambers such that the pressure inside the microphone may always be maintained equal to the water pressure at the outside.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention,

itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of several embodiments thereof, when read in connection with the accompanying drawing, in which Figure 1 is a longitudinal sectional view of one form of my improved microphone.

Figure 2 is a sectional view taken on the line IIII of Figure 1,

Figure 3 is a sectional view of another modification of my invention with reference to which the principle of operation will be explained hereinafter,

Figure 4 is an electrical wiring diagram showing the electrical analogues of the mechanical system shown in Figure 3, and

Figure 5 is a longitudinal sectional view of still another form of my invention.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown, in Fig. 1, a tubular member I of resilient material, such as relatively fiexible rubber, the ends of which are closed on by the closures 3 and 5 which may be made to have fluid-tight connection with the casing I in any suitable manner, as by clamps I. The closure 5 is provided with a plurality of longitudinally extending, peripheral ribs 6 for a purpose hereinafter set forth and may be made fairly massive to insure sinking of the microphone when immersed in water.

Within the casing I is a partition 9 disposed nearer to the closure 5 than to the closure 3 whereby to provide a relatively small chamber II and a relatively large chamber I3 in both of which air is entrapped, a small opening IS in the partition 9 affording communication between the chambers II and I3. Secured to the partition or plate 9, as by screws I1, is a supporting framework I9 in the chamber I I which carries a dynamic or other suitable. translating unit having a vibratory impulse responsive member, such as a diaphragm or the like 2|, the framework I9 being also provided with ribs 22 in extension of the aforementioned ribs 6. A cable 23 which is connected to the translating unit and may be brought out through the opening I5 and through a fluidtight outlet 24 carried by the closure 3 aflords means for providing external connection of the translating unit to uitable amplifying and signal generating apparatus (not shown) A clamp 23 similar to the clamps 3 and 1 may .be provided around the partition 9 and the supporting framework l9 to provide a fluid-tight connection between the plate 9 and the casing I.

The rubber tube l is made of relative'ly soft, flexible rubber and is, therefore, fairly resilient. However, the portion la of the tube I is in engagement with the ribs 6 and 22 and therefore it is, in efiect, considerably stiffer than the portion lb of the tube l which is between th clamps 1 and 23.

Microphonesof this type, according to the prior art, generally embody only a structure corresponding to the casing part la and the apparatus enclosed therein. When such a microphone is immersed to a substantial depth in the ocean,

for example, the pressure of the wateragainst the wall I a causes the latter to collapse against the closure and the framework l9. In such case, vibrations at acoustic frequencies cannot be transmitted to the diaphragm 2! through the air or other fluid in the chamber l I. To prevent co'llapseof the wall la, it has been proposed heretofore to make the casing wall of heavy, stiff rubber. However, this does not solve the problem because a rubber casing which is very stiff will not transmit vibrations at acoustic frequencies with requisite fidelity.

In'th'e case of my improved microphone, the pressure of the water collapses the wall la somesupporting ring 21 and the voice coil 29, the capacitance ca representing the compliance of the air in the chamber 3| bounded by the partition 9, the supporting frame l9 and diaphragm 2l. The inductance ma and the resistance 1'2,

j represent, respectively, the mass and. resistance what, but it also collapses the resilient wall lb to porting frame l9. As a. result, the pressure in the chambers II and l3 becomes equal to the .pressure of the water at the point wher the microphone is located. Hence, the stiffness of the wall or casing portion la remains constant regardless of the depth to which the microphone la is immersed in the water and is always resilient enough to transmit vibrations at acoustic frequencies with good fidelity. The area of the cas-' ing portion or resilient wall la is made much larger than the area of the diaphragm 2|. This tends to match the relatively large acoustic impedance of the water to the smaller acoustic impedance of the moving parts of the translating unit and thevibrations at acoustic frequencies are therefore efliciently transmitted to the diaphragmZL An examination of Figs. 3 and 4 will make more readily apparent how my improved micro phone operates. Let it be assumed that p1 is the pressure of the water on the resi'lient wall la and m is the pressure of the water on the larger resilient wall lb. In Fig. 4, the inductance m1 represents the mass of the wall la and 0A represents the compliance of this wall, 01 being the compliance of theair in the chamber II. The inductance 1122 represents the mass of the diaof the air or other fluid in the opening or passageway l5. The capacitance 04 represents the compliance of the air in the chamber l3, the capacitance C5 represents the compliance of the wall or casing portion lb, and finally, the inductance m4 represents the mass of the wall lb.

From the description of the resilient walls la and lb heretofore set forth, it will be understood that the capacitance C5 is much greater than the capacitance CA. Also, the capacitance 0:; is large compared to the capacitances 0a and c1, and the-impedance offered by the inductance ms and resistance T2 is large compared to the impedance of the circuit to the left of the capacitance 03 (Fig. 4). Hence, since the impedance offered by the opening I 5 to the flow of fluid from the chamber l3 to the chamber II at acoustic frequencies is high, acoustic vibrations reaching the microphone casing will be transmitted to the diaphragm 2| only throu h the wall la and the couplin fluid therebetween in the chamber II,

and not through the wall lb and the fluid in the chamber l3.

In the modification of my invention shown in Fig. 5, I employ two discrete fluid-tight chambers II and I3 which are separated from each other and are connected to one another by a ,fiuid-tight coupling tube 33. The passage 35 through the tube 33 corresponds to the opening l5 in the partition 9 and has the same effect. In all other respects, the microphone of Fig. 5 is similar to that of Fig. 1.

Although I have shown and described several forms of my invention, it will be apparent to those skilled in the art that many other modifications thereof 'are possible. I, therefore, desire that my invention shall not be limited except in so far as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In signal translating apparatus, the combination of means providing a pair of fluid-tight chambers each including a resilient wall, a vibratory impulse responsive member in one of said chambers adapted to be vibrated inresponse to vibration. of the resilient wall of said last named chamber, and means having fluid-tight connection with each of said chambers and including a passageway for the flow from one of said chambers to the other of fluid entrapped within said chambers.

2. In signal translating apparatus, the combination of means providing a relatively large and a relatively small fluid-tight chamber and including a resilient wall for each of said chambers, said small chamber having a relatively stifier resilient wall than said large chamber, a vibratory impulse responsive member in said smaller champhragm 2|, its peripheral supporting ring 21, and

ber adapted to be vibrated in response to vibrations of said stiffer resilient wall, and means having fluid-tight connection with each of said chambers and including a passageway for the flow from one of said chambers to the other of fluid entrapped within said chambers.

3. In signal translating apparatus, the combination of means providing a pair of fluid-tight chambers each including a resilient-wall, one of said chambers being relativelysmall and the other relatively large, a vibratory impulse responsive member in said small chamber adapted to be vibrated at acoustic frequencies in response to corresponding vibration of the resilient wall of said small chamber, and means having fluid-tight connection with each of said chambers and including a passageway for the flow from one of said chambers to the other of fluid entrapped in said chambers, said passageway having such dimensions as to afford a high impedance to the passage of fluid from said large chamber to said small chamber in response to vibrations of the resilient wall of said large chamber at acoustic frequencies. I

4, In signal translating apparatus, the combination of means providing a pair of fluid-tight chambers each including a resilient wall and each having a first fluid entrapped therein, the resilient wall of one of said chambers being stifier than the resilient wall of the other of said chambers, and means having fluid-tight connection with each of said chambers and including a passageway for the flow of said entrapped fluid from one of said chambers to the other, said chambers and passageway providing means being adapted to be immersed bodily in a second fluid to a depth such that the pressure of said second fluid on said resilient walls exceeds that of the first fluid within said chambers whereby to effect partial col lapse of said resilient walls, the resilient wall of said other chamber being adapted to collapse to a greater extent than said stifler wall whereby to force some of the fluid in said other chamber through said passageway into said first-named chamber to equalize the two fluid pressures on said stiffer wall and thereby maintain said last named wall at substantially constant stifiness regardless of the depth to which said first named chamber is immersed in said second fluid.

5. The invention set forth in claim 4 characterized in that said first-named chamber is rela-' tively small and said second-named chamber is relatively large.

6. The invention set forth in claim 4 characterized by the addition in said first-named chamber of a vibratory impulse responsive member adapted to be vibrated at acoustic frequencies in response to corresponding vibrations of said stiffer wall, said other resilient wall being also subject to acoustic frequency vibrations, and

characterized further in that the dimensions of said passageway are such that said passageway offers a large impedance to the flow of said first named fluid therethrough in response to the vibrations of said other resilient wall at the acous- 1 tic frequencies.

'7. The invention set forth in claim 4 characterized by the addition in said first-named chamber of a vibratory impulse responsive member adapted to be vibrated at acoustic frequencies in response to corresponding vibrations of said stifier wall, and characterized further in that the areaof said member and the area of said stiffer wall are so related to each other that the acoustic impedance of said second fluid is substantially matched to the acoustic impedance of said member.

8. The invention set forth in claim 4 characterized by the addition in said first named chamber of a vibratory impulse responsive member adapted to be vibrated at acoustic frequenciesin response to corresponding vibrations of said stiil' er wall. the area of said stifier wall exposed to said second fluid being large compared to the area of said member.

9. In signal translating apparatus, the combination of a resilient casing, means closing the ends of said casing and having fluid-tight connection therewith, a partition within said casing also having fluid-tight connection therewith, said partition being disposed nearer to one end of said casing than to the other whereby to provide a relatively small chamber and a relatively large chamber within said casing, and a vibratory impulse responsive member in said smaller chamber, said partition having an opening therein affording communication between said chambers,

and the dimensions of said opening being such that said opening provides a high impedance to the flow of fluid entrapped within said chambers from one of said chambers to the other of said chambers in response to acoustical vibrations of said casing but permits interchange of fluid between said chambers in response to a. unidirectional biasing force on said casing whereby the pressure-within said casing will equal the pressure outside of said casing.

10'. In signal translating apparatus, the combination of means providing two discrete fluidtight chambers each including a resilient wall, one of said chambers'being smaller than the other, a vibratory impulse responsive member in said small chamber adapted to be vibrated at acoustic frequencies in response to corresponding vibration of the resilient wall of said smaller chamber, and a tubular member connecting said chambers and having a fluid-tight connection with each of said chambers, said tubular member providing a passageway for the flow of fluid entrapped in said chambers from one of said chambers to the other, and said passageway having such dimensions as to afiord a high impedance to the passage of fluid from the larger one of said chambers to said smaller chamber in response to vibration of the resilient wall of said larger chamber at acoustic frequencies.

. HARRY F. OLSON. 

